High-T superconductor and process for preparing it

ABSTRACT

Polycrystalline high-T c  superconductors of the formula M m  E e  RO x , which contain grains which are crystallographically aligned to the greatest possible extent, where M is at least one trivalent element such as a lanthanide element, E is at least one divalent element such as an alkaline earth element and R is at least one transition metal such as Cu, and x denotes the proportion of oxygen, are obtained by substituting a part of the alkaline earth element by a foreign element, preferably an alkali-metal element, which is no longer present in the product after the reaction sintering and sintering except for contents in the ppm to parts per thousand range and brings about the orientation effect. This produces a material which contains a slight deficit of E and optionally M, has an unaltered critical temperature and is substantially more resistant to external agents than equivalent known materials. A post-treatment in a stream of air or oxygen is unnecessary. Single crystals having relatively large dimensions can also be produced in a corresponding manner.

The present invention relates to a high-T_(c) superconductor, inparticular a ceramic superconductor of a type which contains at leastone trivalent element, at least one divalent element, at least onefurther element, in particular a transition metal element such as copperor niobium, and oxygen. Typical representatives of such ceramichigh-T_(c) superconductors are represented, for example, by the formulaeME_(2-y) R₂ O_(x), ME_(2-y) R₃ O_(x) and M₂ E_(3-y) O_(x), where

M is at least one trivalent element such as a lanthanide element,bismuth, or yttrium,

E is at least one divalent element such as an alkaline earth element,and

R is at least one transition element, and

x specifies the proportion of oxygen.

The transition element component R is preferably composed entirely or atleast partly of copper. In the compound named as second above, it ispreferable that 6,2<x<7.2.

The invention further relates to a process for preparing ceramichigh-T_(c) superconducting materials. It is particularly suitable forpreparing high-T_(c) superconductors of the above type but can beapplied quite generally, that is to say, for example, also to theLa-Sr-Nb-O system.

The term "high-T_(c) superconductor" should in this case be understoodto mean superconducting materials whose critical temperature T_(c) isabove 30K.

It is known that the ceramic high-T_(c) superconductors of theabovementioned types have a strongly anisotropic crystal structure andtheir superconducting properties, such as the critical current densityand the critical field strength, are strongly directionally dependentwith reference to the crystal structure. Efforts are therefore made toprepare single crystals which are as large as possible from thematerials mentioned. However, according to the present prior art, fairlylarge volumes of these ceramic superconducting materials caneconomically be prepared only in polycrystalline form and the individualcrystallites or grains of the polycrystalline material have then to becrystallographically similarly aligned if the optimum superconductingproperties are to be exploited.

A publication by Wu and Ruckenstein in MATERIALS LETTERS, Volume 5, No.11.12, October 1987, pages 432-435 discloses that the critical currentdensity and the critical field strength of YBa₂ Cu₃ O₇ is particularlyhigh in the direction of the |001| planes, i.e. of the Cu-O planes andthat these planes can be aligned in polycrystalline material by pressingand subsequent sintering perpendicularly to the pressing direction.

A publication by Omori et al., JAPANESE JOURNAL OF APPLIED PHYSICS,Volume 26, No. 8, August 1987, pages L1421-L1422-L1423 discloses thepreparation of oriented orthorhombic YBa₂ Cu₃ O_(7-x) polycrystals fromtetragonal platelet-type crystals by grinding, pressing and sintering inan oxygen atmosphere.

A publication in Science-Vol. 238 (1987), pages 1655-1656 discloses thatoriented grains of an yttrium-barium-copper oxide superconductor can beprepared by fusion at 1,300° C., controlled cooling and subsequent heattreatment for the purpose of oxidizing. This made it possible toincrease the critical current density by several powers of ten.

Accounts of Chemical Research, 21, No. 1, pages 1 to 7 discloses thedoping of the Ba sites of a YBCO superconductor with alkali-metal ions(K, Rb, Cs), with YBa₂ Cu₃ O_(7-y) being obtained. Replacing the Cu byNi or Co reduced the critical temperature T_(c) considerably.

High-T_(c) superconductors having the formula LaBa₂ Cu₃ O_(7-x) and RBa₂Cu₃ O_(7-x) (orthorhombic; R=Y, Sin, Eu, Gd, Dy, Ho, Tm, Yb, Lu;O<x<0.2) are disclosed in Nature, Vol. 329, 17.9. 1987, pages 227-229.

The known processes for preparing structured or oriented polycrystallineceramic high-T_(c) superconducting materials are complicated,time-consuming and not always readily reproducible. It has hitherto onlybeen possible to prepare single crystals made of high-T_(c)superconductors with relatively small dimensions. Accordingly, theobject of the present invention is to provide a polycrystallinehigh-T_(c) superconducting material containing grains which arecrystallographically aligned to the greatest possible extent and/orcontaining relatively large single crystals, and also a simple andreproducible process for preparing such a superconducting material.

A preferred process for preparing a ceramic high-T_(c) superconductingmaterial according to the invention, in which process the startingmaterials yielding the desired superconducting material, for example acompound of at least one trivalent element (typically a lanthanideelement, bismuth or yttrium), a compound of at least one divalentelement (typically an alkaline earth metal) and a compound of a furtherelement (typically a transition metal such as copper or niobium), inparticular oxides of these elements, or compounds which yield oxides onheating, are mixed and sintered, comprises, according to the invention,using a mixture which additionally contains a compound, in particular aninorganic compound, which vanishes to the greatest possible extent fromthe reaction mixture at the preparation temperature, for examplevolatilizes, is absorbed by the reaction vessel etc. etc. Preferably,the proportion of the divalent element and optionally also of thetrivalent element is reduced, i.e. a reaction mixture is used whichcontains the divalent element, such as the alkaline earth element, in asubstoichiometric amount, preferably 5 to 10%, optionally up to 15%below the stoichiometric amount. The compound of the foreign elementwhich vanishes during heating to the greatest possible extent from thesuperconducting material produced is, in particular, an inorganic metalcompound such as an alkali-metal compound, for example an alkali-metaloxide. The foreign element or the foreign elements may be contained inthe reaction mixture in an amount which is in terms of moles about equalto, or greater than, the deficit of the divalent (and optionally of thetrivalent) element. The molar amount of the foreign element, that is tosay, for example, of the alkali metal, which is introduced in the formof a compound such as the oxide, may amount to up to ten times the molardeficit.

The added material volatilizes to the greatest possible extent duringthe reaction annealing or sintering which is carried out in air oroxygen at customary temperatures, and only traces of the foreign elementin the order of magnitude of ppm to a few parts per thousand, based onthe molar proportion of the divalent element in the finishedsuperconducting material, are left (e.g., parts per million to parts perthousand range of foreign elemen). It was not possible to achieve theorientation effect by solely reducing the proportion of di- and/ortrivalent element in the starting material mixture. The substitution ofa part of the proportion of divalent element (in particular alkalineearth element) by at least one foreign element (in particularalkali-metal element) in the starting material mixture yields the bestresults. It also appears to be advantageous if the ionic radius of theelement of the removable compound is about equal to the ionic radius ofthe substituted divalent element.

The invention is especially suitable for superconductors having "YBaCu"or "123" structure such as yttrium barium cuprate, a potassium compoundsuch as potassium oxide or potassium carbonate preferably being used asremovable or volatilizable compound if the alkaline earth metal isbarium. The invention can, however, also be used in the case of ceramicsuperconducting materials having another structure, for example of thelanthanum strontium cuprate type, bismuth calcium strontium cupratetype, lanthanum strontium niobate type and analogous compounds. If thesuperconducting material contains alkaline earth elements having smallerionic radii than Ba, compounds of alkali metals having smaller ionicradii than potassium are preferably used as volatilizable compounds.

The material obtained in the manner described above by reactionannealing is in general still somewhat porous. In order to prepare acompact material having crystallographically aligned grains, thematerial obtained during the reaction anneal is comminuted, for examplein a mortar or a ball mill, then pressed in a mold and subsequentlysintered. As a result of this additional pressing and sinteringoperation, a virtually completely compact material, which is 95% or moreoriented, is obtained.

The invention provides some important advantages over the prior art:

Above all, molded bodies containing grains crystallographically alignedto the greatest possible extent perpendicular to the c-axis can beprepared. Within the limits of precision of X-ray spectra, a 100%alignment can be achieved. The process is also suitable for forming thinepitaxial layers of high-T_(c) superconducting materials on suitablesubstrates.

The process also makes it possible to prepare relatively large singlecrystals, it is simple and requires relatively little time, and apost-treatment such as annealing is unnecessary.

A further important advantage of the present superconducting materialsis that they are substantially more resistant to external agents such asthe atmosphere, moisture and mineral acids, than the known ceramicsuperconducting materials.

EXAMPLE 1

Preparation of a polycrystalline body containing aligned crystallites

1.733 g of Y₂ O₃,

6.059 g of BaCO₃,

3.6685 g of CuO and

0.1 to 0.5 g of K₂ CO₃

are mixed in acetone and ground in a mortar. The acetone is thenevaporated off and the mixture is heated for three hours at 400° C. in acrucible, then it is again comminuted in the mortar. The mixture is thensubjected to a reaction sintering at 950° C. for 10 to 20 hours in anAl₂ O₃ crucible and then it is cooled to room temperature at a rate of50° C. per hour. Blocks of centimeter size are obtained which arecomposed of individual, highly oriented polycrystalline regions havingtransverse dimensions of about 1 to 3 mm, as detected by X-raydiffraction. The material is superconducting, which was demonstrated bythe Meissner effect.

In order to obtain a compact ceramic, the material is again comminutedin a mortar, then pressed and heated in air at 950° C. for 5 to 50hours, and subsequently cooled.

The material obtained after the first sintering has the formula

    YBa.sub.1.95 Cu.sub.3 O.sub.6.45+0.2.

Intermediate or subsequent sintering in air (10 to 30 hours at 600° C.)increases the oxygen content slightly.

The compact ceramic prepared in the manner described above is alsohighly oriented (approximately 95%). Both the material obtained afterthe first sintering step and also the compact ceramic are substantiallymore resistant to environmental agents than equivalent known materials.At 94 K, the critical temperature T_(c) determined by conductivitymeasurements is the same as that of the equivalent known material YBa₂Cu₃ O₆.5+x ; x>0.

By repeating the heat treatment at temperatures in the order ofmagnitude of 600° C., it is possible to obtain larger single crystalshaving dimensions of approximately 100 μm and above.

Polycrystalline high-T_(c) superconductors of the formula M_(m) E_(e)CuO_(x), which contain grains which are crystallographically aligned tothe greatest possible extent, where M is at least one trivalent elementsuch as a lanthanide element, E is at least one divalent element such asan alkaline earth element and x is less than or equal to 1.5 m+e+1.5,are obtained by substituting a part of the alkaline earth metal by aforeign element, preferably an alkali-metal element, which is no longerpresent in the product after reaction sintering and sintering except forforeign contents found in the ppm to parts per thousand range based onthe content of divalent (alkaline earth metal) element and brings aboutthe orientation effect. This produces a material which has a slightdeficit of E and optionally M, has an unaltered critical temperature andis substantially more resistant to external agents than equivalent knownmaterials. A post-treatment in a stream of oxygen or air is notnecessary.

EXAMPLE 2

Preparation of relatively large single crystals

1.733 g of Y₂ O₃,

6.059 g of BaCO₃,

3.6685 g of CuO and

0.5 g of K₂ CO₃

are mixed and ground as in Example 1. The mixture is heated to 900° C.and then immediately brought to 980° C.-1,000° C. at 20°/h and heldthere for 24-48 h. The preparation is then slowly cooled (5°-10°/h) to900° C. and rapidly cooled (50°/h) from that point to room temperature.The product is a solid, compact block composed of large crystals(typically 2×3×1.5mm). As a result of post-oxidation (24-48 h; 600° C.;O₂ atmosphere), the tetragonal material becomes orthorhombic. Itexhibits (even as an entire block) the Meissner effect in liquidnitrogen and, in addition, also the suspension effect, and has acritical temperature of 90K. Scanning and polarization micrographs showthat the crystalline regions harbor microdomains. The individual mosaicdomains have only a slight misfit in relation to the orientation.

The chemical analysis shows not only a deficit of Ba, but also an excessof yttrium: (Y₀.44 Ba₀.58)₃ Cu₃ O₆.84 is a typical composition of twocrystals. Interestingly, the Y and Ba content (based on Cu₁) add upexactly to 1, which suggests a disorder in the Y-Ba lattice such as canprobably occur only at high temperatures.

It is claimed:
 1. A ceramic molded body consisting of a high-T_(c)superconductor of the formula ME_(2-y) R₂ O_(x), ME_(2-y) R₃ O_(x) or M₂E_(3-y) R₂ O_(x), where M is at least one lanthanide element, yttrium,bismuth or combinations thereof, E is at least one alkaline earth metalelement, R is copper, x is the proportion of oxygen, and 0.05<y<0.3,wherein crystal grains of the ceramic molded body are alignedapproximately 95% perpendicular to the c-axis.
 2. A molded body asclaimed in claim 1, wherein y>0.1.
 3. A molded body as claimed in claim1, whose composition essentially corresponds to the formula

    YBa.sub.1.95 CU.sub.3 O.sub.6.45+0.25.


4. A process for preparing a ceramic high-T_(c) superconductorconsisting of the formula ME_(2-y) R₂ O_(x), ME_(2-y) R₃ O_(x) or M₂E_(3-y) R₂ O_(x), where M is at least one trivalent element of thelanthanide group, yttrium, bismuth or a combination thereof, E is atleast one alkaline earth element, R is copper, x is the proportion ofoxygen, and 0<y<0.3, which process comprises preparing a startingmixture which contains the at least one trivalent element of thelanthanide group, yttrium, bismuth or combination thereof, asubstoichiometric amount of the at least one alkaline earth element suchthat there is a molar deficit thereof, the copper and additionally atleast one alkali metal compound, lead compound or a combination thereofin an amount equal to or up to 10 times the molar deficit, and annealingthe mixture at a preparation temperature of at least about 950° C.wherein the alkali metal compound, lead compound, a combination thereofis removed by the annealing at the preparation temperature.
 5. Theprocess as claimed in claim 4, wherein a mixture is used in which themolar proportion of the alkali metal compound, lead compound, orcombinations thereof is at least equal to the molar deficit of alkalineearth metal.
 6. The process as claimed in claim 4, wherein barium isused as the alkaline earth metal and a compound of potassium is used asthe alkali metal compound.
 7. The process as claimed in claim 4, whereinthe annealed mixture is cooled, comminuted, pressed into a mold and thenannealed again.
 8. The process as claimed in claim 4 wherein the mixtureis annealed in air.